Pipelines used to transport petroleum fluids such as crude oil and natural gas are commonly wrapped with a jacket of insulating material. For example, in Alaska and other cold climates, insulation is provided along pipelines to prevent the rapid cooling of oil and gas fluids, thus providing better transportability of these fluids. In refineries, pipelines transporting hot fluids are insulated in order to protect personnel from the high temperatures.
In insulated pipelines, the insulation retains moisture around the outside of the pipeline, which moisture promotes corrosion. Therefore, proper maintenance of insulated pipelines requires their periodic inspection for corrosion and other potential leak sources. However, the insulation, which serves as a thermal barrier, also serves as a barrier to inspection with many prior art inspection techniques. Removal of the insulation for inspection and rewrapping of insulation after inspection is both time consuming and expensive. The inspection process is complicated further on those pipelines covered with a metal jacket over the insulation. The metal jacket is used to keep cut moisture. The metal jacket is typically provided in two half portions with each portion having flanges for aiding in the retention of the jacket on the pipeline. The two half portions of the jacket are joined together at the flanges which form seams. Water occasionally enters through the jacket seams and travels through the insulation to the pipe where it causes corrosion.
Prior art methods of detecting pipeline corrosion have proven inadequate. For example, pigs with corrosion detection equipment can only be used on pipelines that have locations providing access to the interior of the pipeline; many pipelines lack such locations. Ultrasonic detection methods require removal of the metal jacket and insulation, a lengthy and expensive procedure. Radiography detection methods are potentially hazardous and the equipment is cumbersome, requiring impractical or inconvenient adjacent vehicular support. Furthermore, with radiography methods it is often difficult to distinguish between corrosion pits filled with corrosion products and uncorroded portions of pipe walls. What is needed then is a method of detecting corrosion through insulation and the surrounding jacket, and which method can be practiced with portable equipment.
Electromagnetic probing techniques provide such a method for detecting corrosion through insulation. In the prior art, frequency domain electromagnetic probing techniques are used to detect corrosion in aircraft fuel tanks. Frequency domain electromagnetic probing techniques utilize a small number of frequencies and measure magnitude and phase differentials between the transmitted signals and the received signals. However, because frequency domain techniques, as a practical matter, utilize only a small number of frequencies, the amount of information obtained is inherently limited, thus detracting from the accuracy of the techniques. In addition, the induced field must be measured in the presence of a much stronger primary field, thus reducing sensitivity.
I have invented a method for inspecting pipelines and other types of electrically conductive containers, using transient electromagnetic phenomena. The method is described in my U.S. patent application Ser. No. 07/134,368, filed Dec. 17, 1987, entitled "METHOD FOR DETECTING CORROSION ON CONDUCTIVE CONTAINERS". Transient electromagnetic phenomena are utilized to inspect insulated pipelines without removing the insulation or the metal jacketing. This application describes improvements to the method of inspecting containers such as pipelines which improvements allow increased spatial resolution and increased speed of data acquisition and thus inspection time. This application also describes a method of displaying the data received from the induced current decay in the container walls, which method results in improvements in data analysis.